Colloids

Question 1

Why colloidal particle are like ‘big atoms’?

3 points

Answer 1

1. Easy to see
2. Not limited by QM
3. Can tune interractions between them

Question 2

Lyophobic colloids

+ Example

Answer 2

Dispersed particles do not like the suspended medium (emulsions)

Question 3

Lyophillic colloids

+ Example

Answer 3

Suspended particles like the solvent (gelatin sol)

Question 4

Describe Tyndall scattering

4 points

Answer 4

1. Light gets scattered by suspended particles
2. Particle size is similar to visible light wavelength
3. Blue light gets scaterred more (\propto 1/lambda^4)
4. Red light gets transmitted

Question 5

Floc

Answer 5

Loose agglomeration of particles to form an open aggregate. Reversible aggregation

Question 6

Coagulum

Answer 6

A strong agglomeration of particles to form a dense aggregare. Irreversible aggregation

Question 7

Sedimentation

Answer 7

Aggregate of colloidal particles denser than solvent

Question 8

Creaming

Answer 8

Aggregate of colloidal particles less denser than solvent

Question 9

Polydispercity

Answer 9

Distribution of particle size. Standard dev over mean

Question 10

Electrical double layer

Answer 10

Comnination of Stern layer and the diffuse layer. These screen the electrostatic interractions experience by the charged surface

Question 11

Stern layer

Answer 11

Oppositely chatged ions strongly adsorbed on a charged surface

Question 12

Diffuse layer

Answer 12

The layer aftern Stern layer where there are more oppositely charged ions than in the bulk sollution

Question 13

Critical coagulation concentration

Answer 13

Salt concentration at which ΔG* = 0

Question 14

What happends at medium salt concentration?

Answer 14

Particles achieve a free energy minimum (floc)

Question 15

What happends at low salt concentration?

Answer 15

Repulsions are not screened by salt in-between
Energy barriers is big => stable colloid

Question 16

What happends at high salt concentration?

3 points

Answer 16

• Short range repulsions
• Electrostatic repulsions are screened by the salt
• VDW wins and substances coagulated irreversibly

Question 17

Surfactor

Answer 17

Emulsification agent

Question 18

Describe the suspension method

2 points

Answer 18

1. Mix two immiscibe liquids + surfactor
2. Surfactor sits at the interfaces and stabilises the liquid

Question 19

Suspension method

What else can be used to stabilise the suspension?

Answer 19

Other colloidal particles

Question 20

Pickering emulsion

Answer 20

When emulsification agent consists of solid particles

Question 21

Define condensation method

Answer 21

Formation of colloidal particles from molecules up

Question 22

3 stages of condensation method

Answer 22

1. Nucleation
2. Growth
3. Stabilisation

Question 23

What does the classical nucleation theory suggest?

4 points

Answer 23

• Solid like particles suddenly appear
• Outcome depends on Gibbs free energy of formation of the particle
• Free energy depends on particle size
• Particle redissolves if its radius smaller than the critical radius

Question 24

Ostwald ripening

Answer 24

When too many things have nucleated, smaller particles redissolve to form bigger particles

Question 25

Describe ouzo effect

Answer 25

Formation of milky oil in water emulsion upon addition of water to some liqueurs

Question 26

Explain ouzo effect

Answer 26

• Oil in water emulsions are not stable
• Oil droplets coalesce until complete phase separation is achieved (at macroscopic levels)
• Addition of small amounts of surfactant stabilises the oil droplets

Question 27

What are the 2 effects giving an overall static equilibrium for a colloidal system?

Answer 27

1. Brownian motion
2. Gravity

Question 28

How can the colloidal equilibrium be described?

Answer 28

Boltzman’s distribution

Question 29

What happens to attraction strength as [polymer] increases

Answer 29

Increases

Question 30

What happens to attraction range as coil diameter of polymer increases

Answer 30

Increases

Question 31

What does the presence of polymer in a colloidal mixture result in? Why?

Answer 31

1. Attraction between the colloidal particles
2. Entropy & Osmotic pressure

Question 32

Define

Critical supersaturation

Answer 32

The concentration required to promote the spontaneous formation and growth of solid particles

Question 33

Explain why the nucleation rate and final particle size are sensitive to the
supersaturation.

Answer 33

Increasing the supersaturation lowers the barrier and exponentially increases the rate of nucleation. Faster nucleation rate leads to smaller particle formation

Question 34

If supersaturation (S) is increased from 1.01 to 1.02 what happens to critical radius and deltaG barrier

Answer 34

Critical cluster radius is reduced by a factor of 2, and the barrier height is reduced by a factor
of 4.